Traditionally, the quality of cast steel produced by a continuous casting process is managed using operating indexes. When an abnormality is detected in any operating index, for example, when the amount of slag outflow from the ladle during an interval between charges is larger than a managed value, or when the submerged entry nozzle through which the molten steel in the tundish is poured into a mold has shown a tendency to clog because of the adhesion of nonmetallic oxide inclusions, or when the fluid condition on the meniscus (molten surface) of the molten steel in the casting mold has become asymmetrical about the submerged entry nozzle, then continuous-cast pieces corresponding to the portion where the abnormality was detected are closely examined for quality before being sent to the subsequent rolling process, and cast steel with low cleanliness is downgraded.
Even if the cast steel is not downgraded, the quality examination itself not only imposes a large burden on the work but also decreases the ratio of the cast pieces directly transferred to the rolling process to the total number of cast pieces produced (the direct transfer ratio), thus disturbing the matching between the continuous casting and the rolling process and leading to a substantial increase in production cost.
On the other hand, even when no abnormality is detected in the operating indexes and the cast steel is rolled as originally scheduled, there may be cases in which defects are discovered in the finished steel plates. after rolling. In such cases also, the yield of the finished products decreases, leading to a substantial increase in production cost.
The most commonly practiced methods for estimating the behavior of nonmetallic inclusions in molten steel in the continuous casting process include a simulation using a water model, a model calculation using a simple analytical solution, and even a simulation calculation by a numerical analysis for simulating the motion of fine particles in a turbulent flow. In implementing measures to reduce inclusions in molten steel, the knowledge obtained through these methods has been utilized, and techniques for controlling the molten steel flow in the continuous-casting mold by using novel tundish shapes and electromagnetic forces have been developed and are being implemented commercially.
Furthermore, rapid advances, in recent years, in the computational power of computers has made possible extremely precise estimation of the behavior of nonmetallic inclusions in the continuous casting process, and it is now possible to simulate agglomeration of nonmetallic inclusions and formation of new nonmetallic inclusions in molten steel in a turbulent flow.
However, the simulation for the formation of nonmetallic inclusions is no more than an estimation in a laboratory or on paper, and is conducted only for the purpose of explaining the behavior of nonmetallic inclusions in molten steel samples taken during casting or steel samples taken from cast steel on a macroscopic scale after the continuous casting, or of explaining on a macroscopic scale the effects of the measures or changes in operating conditions effected during operation, and obtaining equipment and operation indexes. Therefore, it has not been possible to apply such simulation to dynamic prediction of the nonmetallic inclusions in the molten steel during casting or of the internal quality of the resulting cast steel pieces.
The reasons are: (1) techniques capable of analyzing the behavior of nonmetallic inclusions with high accuracy have not been available, and it has not been possible to accurately set the conditions for the simulation calculation of their behavior; and (2) the traditional analysis methods have lacked speediness, and if prediction results with high accuracy are to be obtained, considerable time has had to be spent, as a result of which it has been extremely difficult to predict online the behavior of nonmetallic inclusions in cast steel during continuous casting.